A multi-center study of oxacillin resistant S. aureus is in place and should be completed within a year. Phage types have been, and will continue to be used to fingerprint strains of the oxacillin resistant species. We have evidence that strains with oxacillin resistance not reversible by clavulanic acid are few in number, and that oxacillin resistant S. aureus is represented in many U.S. hospitals by a single predominant strain. S. aureus in which oxacillin resistance is reduced in the presence of clavulanic acid, the hyperbetalactamase producers, have another, distinct phage pattern. We also are studying loss rates of oxacillin resistance after the strains are cultured from the treated patient. This phase will include initial sensitivity tests done directly from clinical samples, followed by tests done during serial subcultures. This has important bearing on sensitivity testing of these S. aureus strains, since they begin to lose resistance immediately, on initial cultures.